1. Field of the Invention
The present invention relates to four-wheel drive vehicles comprising a pair of front or rear driven wheels which are driven directly by a prime mover and another pair of front or rear driven wheels which are driven via a hydraulic clutch. A first hydraulic pump is driven by being operatively connected to the pair of driven wheels and a second hydraulic pump is driven by being operatively connected to the other driven wheels. The engagement force of the hydraulic clutch is controllable according to the difference in rotation between the first and the second hydraulic pumps.
2. Description of the Prior Art
The use of a two-wheel drive state using only one pair of front or rear driven wheels during normal travel is known in the art, and when a difference in rotation between the front and the rear wheels occurs both pairs of front and rear driven wheels are driven together so as to enhance the travel stability. If the difference in rotation between the front and the rear wheels is maintained so as to be small all the time, a tight brake phenomenon is caused when turning at a low speed causing the driver to feel uncomfortable and, moreover, it is necessary to increase the strength of the components used for transmitting the driving force between the front and the rear wheels, thus increasing the weight of the vehicle. It is disclosed in Japanese Patent Application Laid-open No. 3-224830 that the front wheels are always driven. When the difference in rotation between the front and the rear wheels is small, the torque transmitted to the rear wheels is made small, and the torque transmitted to the rear wheels is increased according to increases in the difference in rotation, and thus the above-mentioned problems are solved.
However, on a road surface having a low coefficient of friction such as a snow-covered road it is desirable to increase the amount of torque transmitted to the rear wheels even when the difference in rotation between the front and the rear wheels is small, but in the above-mentioned system the amount of torque transmitted to the rear wheels remains small when the difference in rotation between the front and the rear wheels is small, and thus the objective cannot be obtained.